KR100250340B1 - Method for molding thermoplastic resin - Google Patents

Abstract

A method for molding a thermoplastic resin by filling a molten thermoplastic resin into a mold which comprises carrying out the molding of the resin while reducing the solidification temperature of the resin surface which contacts with the mold during the step of filling the resin. To this effect, a gas which has a high solubility in the resin is present in the mold cavity and is absorbed in the resin surface, thereby reducing its solidification temperature. According to this molding method, the state of the mold surface can be faithfully transferred to molded articles.

Description

Molding method of thermoplastic resin

The present invention relates to a molding method of a thermoplastic resin so that the surface state of the mold can be faithfully transmitted to the surface of the molded article.

In molding a thermoplastic resin, the temperature of the mold is usually maintained at a temperature sufficiently lower than the temperature at which the molding resin is solidified. This is necessary to cool the resin material which is very low in thermal conductivity and is in a molten state to a temperature at which the resin can be removed as a molded article in a short time. Moreover, in order to faithfully transfer the surface state of the mold to the resulting molded article, it is necessary to pressurize the resin in a low viscosity state to the mold under high pressure. However, if the mold temperature is lower than the solidification temperature of the resin, the filling of the resin and the solidification of the resin are simultaneously performed, and the resin contacted with the mold at the front of the flow is rapidly cooled and the viscosity is increased, and furthermore, the resin is at the mold surface under low pressure. Since it solidifies in a pressurized state, it becomes difficult for the surface state of the mold to be faithfully transmitted to the formed article. Therefore, in the case of conventional injection molding, appearance defects may occur due to uneven gloss, seam, flow marks or jetting, or coarse transfer of fine holes in a precision molded article such as an optical disc, and moreover, Poor charging can occur in thin walled parts.

In order to improve the transferability of the mold surface, it is necessary to prevent or minimize the solidification of the resin during the filling step of the resin.

In the case of injection molding of a thermoplastic resin or the like, there is always a need to economically improve the delivery of the mold surface without extending the molding cycle. In order to improve the transfer of the mold surface, various methods as exemplified below have been proposed.

1.The method of repeating heating and cooling of the mold surface by alternating passage of fruit or refrigerant through the mold. (Plastic Technology, Vol. 34 (June), 150 (1988), etc.).

2. Optionally heating the mold surface by radio frequency induction heating immediately before molding (USP 4439492, etc.).

3. A method of providing an insulating layer and an electrically conductive layer on the mold surface, and passing a current through the electrically conductive layer (Polym. Eng. Sci., Vol. 34 (11), 894 (1994), etc.).

4. How to radially heat the mold surface. (Gosei Jushi, Vol. 42 (1), 48 (1996), etc.).

5. A method of coating a mold surface with a thermal insulation layer, heating the molding resin itself to heat the mold surface, and performing molding. (USP 5362226, W097 / 04938, etc.).

According to BH Kim's report (Polym. Plast. Technol, Eng., Vol. 25 (1), 73 (1986)), the methods 1, 2 and 3 above, which heat the mold surface with external energy such as electricity immediately before molding. And 4 are called positive control methods, and Method 5 for heating the mold surface with the heat of the molding resin itself without applying external energy is called negative control method.

Both positive control method and negative control method perform molding by heating the mold surface during injection molding. That is, when the injected molten resin is pressed against the wall surface of the mold, the mold surface is heated to a temperature higher than the solidification temperature of the resin, thereby improving the transfer of the mold surface.

The present invention relates to a method of obtaining a target by a mechanism very different from this conventional forming mechanism, which improves the transfer of the mold surface. That is, a method of achieving a remarkable effect by a new idea different from the prior art has been found, and thus, the present invention has been completed.

The well-known document which is somewhat related to this invention is demonstrated.

There is a so-called counter pressure method, which is pressurized by injecting a pressurized gas into the mold cavity prior to filling the resin, thereby performing injection molding of a foaming agent or foamable resin containing water. It is a method of avoiding surface defects, such as a tornado mark on a molded article produced by blowing of gas. According to this method, a gas pressure is applied to the mold pupil in advance to prevent the occurrence of surface defects due to the rupture of the foam produced by blowing gas or water vapor into the entire flow surface of the molten resin flowing through the mold pupil. The gas used in this case may be one that does not cause quality deterioration of the resin due to oxidation, and air is generally used, and most inert gases can be used in this molding method. This back pressure method is used for injection molding of resins containing blowing agents or insufficiently dried resins.

When the back pressure method is used for the molding of a general non-foamable resin, only the problem of flow, ie, gas present in the pupil enters between the molten resin and the mold and hinders the transfer, or in the case of a gas such as air, the air in the pupil In the portion compressed by, only a problem occurs that the resin is present at a high oxygen concentration at a high temperature and deterioration of the resin due to oxidation occurs. Therefore, there is no effect of improving the transfer of the mold surface. Therefore, in order to accurately and faithfully convey the condition of the mold surface to the resulting molded article, the mold must be opened slightly only upon filling of the resin to release air in the pupil or to reduce the pressure in the mold by a vacuum pump.

JP-A-62-231715 discloses a method of injection molding a water-containing polymer alloy using a back pressure method, referring to inert gases such as air, nitrogen and carbon dioxide as the gas used for prepressurization of the mold pupil. However, it does not suggest the idea of the present invention as described below.

In addition, JP-A-61-213111 has two kinds of molds which replace the internal atmosphere of the mold cavity with carbon dioxide at atmospheric pressure and then perform molding to reduce the voids generated by the air blended into the resin during filling of the resin. A reaction injection molding method is disclosed which involves mixing monomers of a compound and injection molding the mixture. However, reaction injection molding in which the mold temperature is higher than the temperature of the raw material mixture of two or more monomers is very different from the injection molding method of the thermoplastic resin according to the present invention and the technical field, and thus the prior art solidifies the resin during the filling step of the resin. It does not disclose a method of improving the poor transferability of the mold surface caused by.

Meanwhile, J. Appln. Polym. Sci., Vol. As shown in many literatures, such as 30, 2633 (1985), when carbon dioxide is absorbed into a resin, it acts as a plasticizer for the resin and lowers the glass transition temperature, but it is widely used in molding resins. It doesn't happen. As one of the few examples, DE-A-4314869 discloses dissolving carbon dioxide or hydrocarbons in a supercritical state in a bioabsorbable polyester in a high pressure vessel to reduce the glass transition temperature, and molding the resin at a low temperature of about 50 ° C. A method is disclosed. However, since this method causes a reduction in the glass transition temperature of the entire resin, for molding, it is necessary to use a mold temperature lower than the normal temperature by decreasing the glass transition temperature, and thus caused by solidification during filling of the resin. There is no effect to prevent the poor quality of transmission.

It is an object of the present invention to economically provide a molding method of a thermoplastic resin in which the state of the mold surface is faithfully transmitted to the molded article by preventing the resin from solidifying or increasing in viscosity during the filling of the resin.

As a result of conducting the research by the present inventors in order to achieve this object, the inventors have found a method that can faithfully transfer the state of the mold surface to the molded article in a manner very different from the conventional method of improving the transfer of the mold surface by heating. Thus, the present invention has thus been completed. That is, the present invention includes the following method:

1. A method of molding a thermoplastic resin by filling the mold with a molten thermoplastic resin, comprising performing molding of the resin while reducing the surface and solidification temperature of the resin in contact with the mold during filling of the resin.

2. The molding method according to the above 1, wherein the thermoplastic resin is an amorphous resin and the solidification temperature is a glass transition temperature.

3. The molding method according to 1 or 2 above, wherein the thermoplastic resin is filled into a mold cavity filled with a gas having a solubility in the thermoplastic resin at least twice as high as air and / or nitrogen at the solidification temperature of the resin.

4. The molding method according to the above 3, wherein the gas is carbon dioxide.

5. At the solidification temperature of the resin, at least 0.1% by weight of the gas is dissolved in the resin so that the gas is present in the mold cavity, and then molten resin is filled into the mold cavity to perform molding. Molding method.

6. At the solidification temperature of the resin, at least 0.5% by weight of the gas is dissolved in the resin so that the gas is present in the mold cavity, and then the molten resin is filled into the mold cavity to perform molding. Molding method.

7. The molding method of 1, 2, 3, 4, 5 or 6, wherein the molding is injection molding.

1 is a diagram showing the solubility of carbon dioxide in polystyrene.

2 is a diagram showing the solubility of nitrogen in polystyrene.

3 is a diagram showing the solubility of carbon dioxide in polystyrene.

4 shows the solubility of carbon dioxide in polystyrene.

5 shows a decrease in Tg due to the dissolution of carbon dioxide in polystyrene.

6 shows the solubility of carbon dioxide in the PMMA / PVF ₂ polymer alloy.

7 shows a decrease in Tg due to the dissolution of carbon dioxide in the PMMA / PVF ₂ polymer alloy.

8 shows the solubility of carbon dioxide in polycarbonate.

9 is a diagram showing the solubility of carbon dioxide in polysulfone.

FIG. 10 shows a decrease in Tg due to dissolution of carbon dioxide in each synthetic resin.

11 is a cross-sectional view of a nozzle portion of an injection molding machine for carrying out the present invention, which is directly related to the present invention.

12 (a) is a cross-sectional view of the entire mold for carrying out the present invention, which is directly related to the present invention.

Figure 12 (b) is a plan view of the moving side of the mold.

Fig. 12 (c) is a detailed sectional view of the circumference of the mold cavity.

Fig. 12 (d) is a detailed sectional view of the sealing portion of the protruding pin.

13 is a diagram showing the configuration of a gas supply device used in carrying out the present invention.

This inventor paid attention to the gas in the mold cavity which was thought to interfere with the transfer of the mold surface, and considered the expression process of the effect as follows.

In injection molding, the resin always flows as a layered fluid in the mold cavity, in contact with the cooled wall surface of the mold at the interface to form a solidified layer, after which the filled resin flows and into the interior of the solidified layer. Advance and after it reaches the front of the flow, it flows towards the wall surface of the mold in a manner called so-called fractional flow. When the pupil is filled with a particular gas, such as carbon dioxide under a suitable pressure, and the resin is filled into the mold cavity, the gas is absorbed into the flow front of the flow resin, or enters the interface between the mold and the resin, and the surface layer of the resin Is dissolved in. The gas dissolved in the resin acts as a plasticizer, selectively reducing only the solidification temperature of the resin surface or reducing the melt viscosity of the resin. If only the solidification temperature of the thin resin surface layer decreases and reaches a temperature lower than the mold surface temperature, solidification does not occur in the step of filling the resin, and as a result, the transfer of the mold surface to the molded article can be significantly improved. . The gas dissolved in the resin surface layer diffuses into the resin over time, and the solidification temperature of the resin surface layer increases. Therefore, the surface layer solidifies within the normal resin cooling time, and the molded article can be separated.

As a result, the present invention was completed in such a manner that molding was carried out while reducing the solidification temperature of the resin surface in contact with the mold during the filling step of the resin.

The resin used in the present invention is a thermoplastic resin available for general injection molding. Preferred are amorphous thermoplastics, thermoplastic polymers mainly composed of amorphous resins, and some crystalline thermoplastics with low crystallinity. Particularly preferred are water styrene resins such as polystyrene, styrene-acrylonitrile copolymers, rubber reinforced polystyrene, styrene-methyl methacrylate copolymers, ABS resins and styrene-methyl methacrylate-butadiene copolymers; Methacryl resins such as poly methyl methacrylate and methyl methacrylate-styrene copolymers; Polyvinyl acetate; Polycarbonate; Polyphenylene ethers; Modified polyphenylene ethers containing polystyrene; Polysulfones; Polyether sulfones; Polyether imides; Polyarylates; Polyamideimide; And vinyl chloride resins such as polyvinyl chloride, vinyl chloride-ethylene copolymer and vinyl chloride-vinyl acetate copolymer. Also included are mixtures of these resins, these amorphous resins containing part of the crystalline resin and resins containing various inorganic or organic fillers.

In the present invention, a combination of a resin in which gas is well dissolved and a gas is preferable. When using carbon dioxide as a gas, a larger effect can be obtained by using a resin having a high affinity for carbon dioxide and a high solubility for carbon dioxide. Moreover, in the present invention, a great effect can be obtained even in the case of a resin which is hard to be processed to provide a molded article having a poor appearance.

In the present invention, the solidification temperature of the resin is the temperature at which the molten thermoplastic resin solidifies in the mold, which is the glass transition temperature of the amorphous resin and the crystallization start temperature of the crystalline resin. In the case of an incompatible polymer alloy, the solidification temperature is the glass transition temperature or crystallization start temperature of the resin, which forms a sea island structure. Here, the crystallization start temperature of the crystalline resin is a temperature at which heat generation due to crystallization of the resin first occurs when the resin is heated to a molding temperature at which it is melted, and this temperature is then 20 ° C / s using a differential calorimeter. Cooled at the rate of minutes.

The gas filled in the mold cavity is a gas having a high solubility in the thermoplastic resin, that is, a gas having a solubility twice as high as air and / or nitrogen at the solidification temperature of the resin and having a plasticizing effect on the resin. That is, gas is present in the mold cavity and is absorbed by the resin surface during the filling process of the resin to reduce the solidification temperature of the resin surface in contact with the mold. As is known, gas with solubility in resin, similar to air or nitrogen, only prevents the transfer of the mold surface in the pupil, and the gas used here needs to have twice as much solubility as air or nitrogen. In addition, the gas is selected under the constraint that it should not degrade the quality of the resin, not adversely affect the mold or molding environment, and should be inexpensive. If solubility is high, a mixture of two or more gases can be used. Examples of the gas include carbon dioxide, hydrocarbons such as methane, ethane and propane, and floons obtained by substituting some hydrogen in hydrocarbons with fluorine or the like. Among them, appropriate ones are selected according to the kind of thermoplastic resin used. Carbon dioxide can be used as the most appropriate in terms of stability, price and ease of handling, and it also dissolves very well in the resin and acts as a plasticizer, resulting in a great effect of reducing the solidification temperature of the resin.

Reducing the glass transition temperature (hereinafter abbreviated as “Tg”) of the resin due to the solubility of carbon dioxide and dissolution of carbon dioxide in the resin, which is most suitably usable in the present invention, will be explained with reference to the accompanying drawings.

1 to 10 show the report results of various documents. 1 and 2 are referred to in “Seikei Kakou, '96 (JSPP'96 Tech, Papers), 279 (1996), and FIG. 3, 4, 5, 6 and 9 says, “J. Appl. Polym. Sci. ”, Vol. 30, 4019 (1985), and FIGS. 7 and 10 describe “J. Appl. Polym. Sci. ”, Vol. 30, 2633 (1985), and FIG. 8 states “J. Membrane Sci. ”, Vol. 5, 63 (1979).

1 and 2 show the solubility of carbon dioxide and nitrogen in polystyrene, with carbon dioxide having about 10 times solubility compared to nitrogen.

3 and 4 show the solubility of carbon dioxide in polystyrene containing liquid plasticizers, and FIG. 5 shows a decrease in Tg due to the dissolution of carbon dioxide. The Tg of polystyrene can be easily reduced by dissolution of carbon dioxide.

6 and 7 show the decrease in Tg due to the solubility of carbon dioxide and the dissolution of carbon dioxide in polymethyl methacrylate and polyvinylidene fluoride polymer alloys. Tg can be easily reduced by dissolution of carbon dioxide.

8 and 9 show the solubility of carbon dioxide in polycarbonates and polysulfones.

Figure 10 also shows the reduction in Tg of each resin due to the dissolution of carbon dioxide. The reduction in Tg due to the dissolution of carbon dioxide is almost the same for the resin except for polycarbonate. In the case of polycarbonates, the reduction in Tg due to the dissolution of carbon dioxide is very large.

With respect to the pressure of the gas trapped in the mold cavity, with the increase in pressure, a larger amount of gas is dissolved in the resin, and the solidification temperature is lowered, so that solidification during filling of the resin can be suppressed even at a lower mold temperature. have. In practice, the required gas pressure depends on the desired degree of delivery of the mold surface, the type of resin or gas, the mold temperature and the like. When gas of high solubility is used and the mold temperature is fixed high, sufficient transferability can be obtained even at low gas pressure.

The lower limit of the pressure is determined by the effect of the gas dissolved in the resin as a plasticizer, and the pressure under the condition that the gas is dissolved in an amount of 0.1% by weight in the resin at equilibrium at the solidification temperature of the resin, preferably 0.5% by weight. The pressure under conditions at which% gas is dissolved. Here, the solubility of the gas in resin is a numerical value measured by the pressure drop method. Even if the pressure is less than the minimum or the pressure below the atmospheric pressure, if a gas having a high solubility, such as carbon dioxide, is used, an effect of improving the transferability which is greater than or equal to that obtained when the pressure in the pupil is reduced by a vacuum pump can be obtained. When low pressure is used, it is desirable to replace the internal atmosphere in the pupil with a specific gas as much as possible.

The upper limit of the pressure is not particularly limited, but if it is too high, the opening force of the mold cannot be ignored or the sealing of the mold becomes difficult. In view of these problems, 15 MPa or less, preferably 10 MPa or less is practical. The pressure of the gas is preferably as low as possible to achieve the desired effect in order to minimize the amount of gas used in one injection and to simplify the sealing of the mold and the structure of the gas supply device. .

The air remaining in the mold upon closing of the mold is preferably replaced by the gas used during or after the mold clamping. However, if the gas pressure used exceeds 1 MPa, the influence of air can be almost neglected.

After the mold pupil is filled with resin, the gas forced out of the pupil is released and set to atmospheric pressure. Release of the gas is performed after filling the mold pupil with molten resin. After the pupil is filled with the resin, it is preferable to apply sufficient pressure to the resin in the pupil until the surface of the molded article is solidified for delivery to the molded article in the mold surface state. In particular, when the dot-shaped groove structure on the mold surface is transferred, it is necessary to pressurize the resin into the mold against the gas pressure in the groove, and in this case, the molding is performed under a higher resin pressure than in normal molding. It is desirable to.

The gas dissolved in the resin is gradually released when the molded article is left after molding the resin. There is no bubble generation in the molded article due to the release of the gas, and after the gas is released, the mechanical performance of the molded article is no different from that produced by the conventional method.

Preferably, several methods of preventing liquefaction of gases are contemplated for devices for supplying gas to the pupil and for discharging gas from the pupil, gas piping and molds, and the like. This means that not only the gas pressure can be obtained at the temperature at which the gas is liquefied, but also when the liquefied gas is in contact with the resin in the pupil, a large amount of gas is dissolved in the resin, and after the gas is released, the surface of the molded product is foamed, This is because the appearance becomes poor. As a liquefaction prevention method, the following method can be mentioned. That is, the gas is heated with a heater and the temperature of the gas flow path and the mold is maintained above the critical temperature of the gas; Providing a pressure release valve capable of maintaining the gas pressure in the pupil and the pipe in an arbitrary range to suppress a sudden increase in pressure caused by the gas forced out of the pupil during filling of the resin; A gas reservoir is installed to allow gas to flow in the reverse direction from the pupil. However, it is undesirable to raise the temperature of the gas excessively in order to suppress the liquefaction of the gas because the amount of gas in the pupil decreases due to the expansion of the gas.

In back pressure molding, for example, to create a hermetic structure of a mold, a hermetic seal is formed by sealing the split face and the plate, and also covering the entire portion of the protruding pin plate to which the protruding pin is fixed, such as a protruding pin that connects the pupil with a 0-ring. The method is usually used. When the 0-ring is used for sealing the protruding pin, the protruding pin must be inserted after the 0-ring is placed between the two plates. In this case, if the resistance against the introduction of the pin is large, the 0-ring is damaged or the 0-ring is deformed by the edge of the tip of the protruding pin, and in many cases a reliable seal cannot be maintained. On the other hand, if a rubber packing (hereinafter abbreviated as “U-packing”) having a U-shaped cross section in the radial direction is used for sealing, there is little introduction resistance at the time of introduction of the protruding pin and damage by the edge of the tip of the pin. Or the mold can be easily assembled without deformation, and thus a high releasable seal can be obtained.

In addition, when the movable pin is sealed with the packing, pressurized gas entering the space around the pin between the pupil and the packing is collected in the space by the filling of the resin, and when the molded article cools and leaves the mold surface, the gas enters the pupil. It flows and sometimes dents into the surface of the molded article that is not yet sufficiently solidified, or expands or deforms the molded part upon mold opening. When this problem occurs, a channel or a hole is provided in the mold through which the gas entering the space around the fin through the non-pupil passage can be discharged out of the mold, and after the pupil is filled with the resin, the gas pressurized out of the pupil At the same time, it is preferable to perform degassing. 12 (a) and 12 (c) illustrate an example of the structure of a mold in which pressurized gas can be discharged through a passage other than a pupil.

Injection of gas into the pupil is possible when the mold structure is generally used for evacuation of the pupil. For this purpose, a slit mounted on the dividing surface on the circumference of the pupil, a space around the pupil insertion block or the protruding pin, a venting pin, a linear body made of a porous sintered body, or the like can be used. When the atmosphere of the pupil is replaced with a gas of about atmospheric pressure, there is a need for an economical way in which the air in the pupil can be replaced with as little as possible, in particular if possible 100% of the gas, in the shortest possible time. A suitable method is to blow gas into the pupil through the mold spout. By injecting gas from the mold spout before filling the resin into the pupil, the gas is pushed out by the resin, and as a result, the resin is molded while air remaining in the pupil is discharged out of the mold by the gas. That is, the atmosphere of the snout, the runner and the gate of the mold are sufficiently replaced by gas, and this gas which is in contact with the resin is always the injected gas.

11 shows a nozzle for injecting a gas to pressurize the pupil from the snout portion of the mold. In FIG. 11, the nozzle 2 connected to the injection cylinder 1 has a needle valve 4 which opens and closes the nozzle tip 3. The outer nozzle 5 is mounted to the nozzle tip portion, and the space 6 formed by the niche body 2 and the outer nozzle 5 is connected to the gas supply source through the passage 7. When the outer nozzle 5 is in light contact with the mold, the space 6 is connected to the pupil, and in this state, gas is injected into the mold from the space 6. Subsequently, when the injection cylinder 1 advances forward and strongly presses the external nozzle 5 corresponding to the mold, the spring pressurizing the external nozzle 5 corresponding to the mold is compressed, and the nozzle body 2 proceeds forward to between the space 6 and the mold. Block the connection. In this state, the resin is filled from the injection cylinder 1 into the mold.

The present invention also includes a method characterized in that the molding is carried out by filling gas into the mold cavity under a low pressure of about 1 MPa from atmospheric pressure, and then filling the pupil with molten resin to compress the gas in the pupil to increase the gas pressure. . When using a mold having a structure in which the gas in the pupil is sealed with a 0-ring or the like, and filling the pupil with gas under atmospheric pressure to a low pressure of about 1 MPa, when the resin is filled, the gas is compressed by the resin, and the gas pressure is Increases as charging progresses. When the gas pressure increases, the amount of gas dissolved in the resin increases, and the resin is plasticized by the dissolved gas so that the fluidity is improved, and as a result, the transferability of the mold surface can be greatly improved. In the case of a typical injection molded article, the transferability of the mold surface at the distal end of the resin flow, which is inferior to the injection pressure injection, is inferior to the gate portion, while the transferability of the mold surface at the distal end of the resin flow is according to the above method. Can be improved.

The method is also effective for the transfer of fine grooves present on the mold surface. In many cases, the resin cannot sufficiently enter the inside of the fine grooves by solidification of the resin in the flow or air trapped in the grooves. However, according to the present invention, since the collected gas is absorbed into the resin, it hardly interferes with the filling of the resin, and the solidification temperature of the resin is reduced and the fluidity is increased due to the plasticizing effect of the absorbed gas. Thus, the resin can be filled into the innermost of the grooves.

The present invention also provides another molding method in which the effect of transferring the mold surface under low gas pressure in the pupil can be improved. That is, the method includes performing a molding while reducing the solidification temperature of the surface of the resin during the filling step by allowing the liquid dissolved in the resin and acting as a plasticizer to be present at the interface between the mold and the molten resin in contact with each other. The transferability of the mold surface to the molded article can be improved by appropriately selecting a plasticizer and coating it on the mold surface at an appropriate thickness.

The present invention also includes a molding method comprising injecting carbon dioxide or the like into a cooled mold cavity with vapor and / or mist of a liquid in which carbon dioxide or the like is easily dissolved. Here, the liquid dissolves very well in carbon dioxide, has a boiling point higher than the mold temperature, and dissolves well in the resin. What can be used suitably is a good solvent or plasticizer for resins with high solubility in carbon dioxide. In general, water, ketones such as acetone and methyl ethyl ketone, alcohols such as ethyl alcohol and various polar solvents can be used. A large amount of carbon dioxide, which plasticizes the resin due to droplet condensation on the cooled pupil surface by injecting carbon dioxide containing vapor and / or mist of liquid carbon dioxide is easily dissolved, into the cooled mold pupil The pupil surface is coated with a thin layer of the containing liquid, and the resin surface layer is impregnated with a large amount of carbon dioxide by pressing the resin on the surface during molding, whereby the transferability of the mold surface to the molded article can be improved. That is, this method involves supplying a large amount of carbon dioxide to the resin surface while supplying a liquid containing a large amount of carbon dioxide on the mold surface, while only supplying low pressure carbon dioxide to the pupil. The thickness of the liquid thin film on the mold surface must be within a range such that the resin surface does not fall off from the mold surface upon filling of the resin. In general, the thickness is preferably in the range of about 0.1-10 탆. The concentration of the liquid in carbon dioxide is preferably such as to provide a liquid thin film of the above thickness.

In the present invention, various injection molding methods can be used satisfactorily. Generally low pressure injection molding methods known to be inferior in terms of transferability of the molding surface, such as gas assisted injection molding, liquid assisted injection molding and injection compression molding, can be used satisfactorily. Moreover, an injection molding method including low-speed filling of the resin, in which the movement speed of the melt front end is 200 mm / sec or less, in particular 100 mm / sec or less, can be satisfactorily used in the pupil. This includes the case where the flow rate of the resin is temporarily low, the flow stops momentarily, the flow rate continues to be low, and the like. According to the present invention, since solidification of the resin may be hindered during the filling of the resin, due to the difference in the flow rate of the resin, a mold surface transfer called hesitation marks, which is often seen in gas assisted injection molding, is used. There is little partial difference in.

Moreover, the method of the present invention can be used in combination with a conventional method of raising the mold surface temperature to improve mold surface transferability. In this conventional molding method, the mold temperature is high, and the resin and the mold tend to stick to each other when the resin is filled, and when air in the pupil is collected between the resin and the mold, this often forms grooves on the resin surface. By using this method together with the present invention, not only the formation of grooves on the resin surface can be avoided, but also high mold surface transferability can be obtained even at a low mold temperature, and the heating efficiency can be improved.

Furthermore, the method of the present invention can be used in combination with a method of vibrating the resin during the filling step of the resin, thereby producing a molded article having not only good mechanical properties but also high transferability of the mold surface. As a method for the vibration of the resin, the following method may be mentioned: a method of vibrating the resin in the injection cylinder (Polm. Plast. Technol, Ehg., 17 (1), 11 (1981), etc.); Vibrating the mold (“Seikei Kakou'97 (JSPP'97 Tech. Papers), 185 (1997), etc.); Vibrating pressurized gas in the pupil (Plastics World, July 8 (1997), etc.). In particular, when the method of the present invention is used in combination with a method of vibrating a pressurized gas in a pupil, the disturbance of transmission due to nitrogen conventionally used is suppressed, resulting in a very high synergistic effect.

According to the present invention, the state of the mold surface can be economically and very faithfully transmitted to the molded article. Therefore, when the appearance of the molded article is poor, subsequent steps such as poorly performed coatings are not necessary, and the part cost is significantly reduced, and in addition, the flat lens manufactured by pressure molding having a lower yield than injection molding The yield can be significantly improved because the former method cannot uniformly transfer the fine orientation of the mold surface to the molded article. Thus, new applications of injection molding can be expected.

As a molded article that can be satisfactorily produced by the present molding method, there may be mentioned injection molded articles such as parts of optical instruments, exteriors of light conducting devices and electronic devices, commercial and office equipment, various automobile parts, various household articles, and the like. The method is suitable for improving the appearance of electronic parts, electrical parts, and exteriors for commercial and office equipment, which are injection molded by multi-point gates to provide the appearance and pattern engraving moldings of many seam and mat shaped parts. Moreover, the present method is a lens manufactured by molding various optical components, such as transparent synthetic resin, such as lenticular lenses and Fresnel lenses, recording disks such as optical disks, and liquid crystal display device components such as optical. It is suitable for manufacturing injection molded articles such as guide plates and diffuser panels. The effect of the present invention on the molded article produced by the method is to improve the transferability and gloss of the mold surface, to reduce the poor appearance due to the seam and to improve the reproducibility of the sharp edges of the mold surface and fine irregularities on the mold surface. A further effect is the improvement of the frying performance due to the reduction of internal stresses generated near the surface of the molded part upon filling of the resin, the reduction of birefringence, the improvement of chemical resistance and the decrease in the orientation of the added rubber. Moreover, since the generation of gas from the entire surface of the melt is suppressed in the filling step of the resin by embedding a high pressure gas in the pupil, it is possible to reduce the stain of the mold and the required amount of power for separating the molded article.

The effects of the present invention will be further explained by the following examples and comparative examples.

Resin used for injection molding is rubber-reinforced polystyrene (STYRON 400, manufactured by Asahi Kasei Kogyo KK), ABS resin filled with 20% glass fiber (STYRAC ABS R240A, manufactured by Asahi Kasei Kogyo KK), and methacrylic resin (DELPET). 80NH, manufactured by Asahi Kasei Kogyo KK, and polycarbonate (PANLITE L1225, manufactured by Teijin Kasei Co., Ltd.).

Carbon dioxide having a purity of 99% or more is used as the gas.

The molding machine used was SG50 (Sumitomo Heavy Industries Ltd.).

The molded article is a rectangular flat plate 2 mm thick and 100 mm x 100 mm. The structure of the mold is shown in FIGS. 12 (a)-(d) and the structure of the gas supply device is shown in FIG. As for the mold surface, half of the pupil surface on the moving side is satinized, and the other half is a specular surface. The direct gate, 8 mm in diameter, is mounted in the center of the molded part, the length of the spout is 58 mm, and the diameter of the nozzle contact is 3.5 mm. A hole 10, which is connected to the outside of the mold through a slit 8, a gas flow channel 9, and a gas flow channel 9 with a depth of 0.05 mm, is mounted on the circumference of the mold cavity for supply and discharge of gas. The mold is connected to the gas supply device through the hole 10, and the 0-ring 11 is mounted near the vent slit and the gas sealing hole to seal the pupil. Moreover, the protruding pin 12 is sealed by inserting the U-packing 15 between the pupil block 13 and the support plate 14. The U-packing used is the MPR series (Nippon Valqua Industries, Ltd.). The hole 10 connected to the outside of the mold is also connected to the space around the protruding pin 12 and the space between the pupil block 13 and the support plate 14, so that the gas in the space can be released at the same time as the filling of the resin is completed.

In the gas supply device, Bomb 16, filled with liquefied carbon dioxide gas and maintained at 40 ° C., is used as the gas source of about 12 MPa. The gas is supplied from bomb 16 through heater 17 and regulated to a given pressure by reducing valve 18 and then stored in a gas reservoir 19 having a dot product of 100 cm 3, maintained at about 40 ° C. The supply of gas to the mold cavity is performed by opening the supply solenoid valve 20 mounted downstream of the gas reservoir 19 and simultaneously closing the discharge solenoid valve 21, the gas reservoir and the pupil being connected to each other during filling of the resin. Simultaneously with completion of the filling of the resin, the supply solenoid valve 20 is closed, and the discharge solenoid valve 21 is opened to discharge gas out of the mold. After the supply of the gas, when the pressure is increased by filling the molten resin to compress the gas into the pupil, the supply solenoid valve 20 is closed at the same time as the filling of the resin, and the discharge solenoid valve 21 is closed when the filling of the resin is completed. Open. Unnecessary increase in pressure during filling of the resin is suppressed by releasing gas from the pressure relief valve 22.

The transferability of the state of the mold surface is evaluated by measuring the surface gloss of the mirror surface portion, observing with an optical microscope, and measuring the surface roughness of the easy-ized portion. Each change polisher UGV-5K (manufacturer: Suga Shikenki Co., Ltd.) is used for the measurement of surface gloss, and SURFCOM 575A (manufacturer: Tokyo Seimitsu Co., Ltd.) is used for the measurement of surface roughness.

Example 1

A mold having a pupil surface temperature of 70 ° C. is filled with carbon dioxide under a pressure of 5.0 MPa, and rubber-reinforced polystyrene having a resin temperature of 220 ° C. is filled with a filling time of 0.6 seconds to 2.4 seconds. The pressure of the resin in the cylinder of 35 MPa is maintained for 10 seconds, and the resin is cooled for 20 seconds. Next, the molded article is separated. The carbon dioxide charged in the mold is discharged to the atmosphere at the same time as the filling of the resin is completed.

The surface gloss of the resulting molded article was measured to find that the surface gloss was excellent regardless of the filling time (60 ° mirror gloss = 101 for both samples).

Example 2

A molded article was obtained in the same manner as in Example 1 except that the pressure of carbon dioxide charged in the mold was 2.5 MPa.

The surface gloss of the resulting molded article was measured and found to be excellent in surface gloss regardless of the filling time (60 ° mirror gloss = 88 for both samples).

Example 3

A molded article was obtained in the same manner as in Example 2 except that the mold pupil surface temperature was 80 ° C.

The surface gloss of the resulting molded article was measured to find that the surface gloss was excellent regardless of the filling time (60 ° mirror gloss = 108 for both samples).

Example 4

A molded article is obtained in the same manner as in Example 1, except that the ABS resin filled with 20% glass fiber is used, the mold pupil surface temperature is 88 ° C, the resin temperature is 240 ° C, and the holding pressure is 70 MPa.

The surface gloss of the resulting molded articles was measured to find that the surface gloss was excellent regardless of the filling time (60 ° mirror gloss = 99 for both samples).

In addition, the surface of the molded article was observed under a microscope of 100 times to confirm that there was practically no glass fiber exposed on the surface, and that the surfaces of both molded articles were smooth.

Example 5

A mold having a pupil surface temperature of 80 ° C. was filled with carbon dioxide at a pressure of 5.0 MPa, and a methacrylic resin having a resin temperature of 240 ° C. was filled with a filling time of 0.6 seconds. The pressure of the resin in the cylinder of 80 MPa is maintained for 10 seconds, and the resin is cooled for 20 seconds. Next, the molded article is separated. The carbon dioxide charged in the mold is discharged to the atmosphere at the same time as the filling of the resin is completed.

The resulting molded article had a surface roughness Rmax of 12.0 μm in the easily segmented portion.

Example 6

A mold having a pupil surface temperature of 120 ° C. was filled with carbon dioxide at a pressure of 5.0 MPa, and a polycarbonate resin having a resin temperature of 300 ° C. was filled with a filling time of 0.6 seconds. The pressure of the resin in the cylinder of 120 MPa is maintained for 10 seconds, and the resin is cooled for 20 seconds. Next, the molded article is separated. The carbon dioxide charged in the mold is discharged to the atmosphere at the same time as the filling of the resin is completed.

The resulting molded article has a surface roughness Rmax of 11.5 μm in the easy-to-part.

Comparative Example 1

A molded article was obtained in the same manner as in Example 1, except that the mold was opened to the atmosphere without connecting the gas supply device.

The surface gloss of the resulting molded article was measured. It was found that 60 ° mirror gloss was 61 when the filling time was 0.6 seconds and 48 when the filling time was 2.4 seconds, and the surface gloss of this molded product was inferior and dependent on the filling time.

Comparative Example 2

A molded article was obtained in the same manner as in Example 1 except that nitrogen was used as the gas charged in the mold.

The surface gloss of the resulting molded article was measured. As a result, the molded article was found to be inferior in surface gloss than in Comparative Example 1. (60 ° mirror gloss = 46 for a charging time of 0.6 seconds, and 60 ° mirror gloss = 40 seconds for a charging time of 2.4 seconds).

Comparative Example 3

A molded article was obtained in the same manner as in Example 4, except that the mold was opened to the atmosphere without connecting the gas supply device.

The surface gloss of the resulting molded article was measured. It was found that the 60 ° mirror gloss was 85 when the filling time was 0.6 seconds and 62 when the filling time was 2.4 seconds, and thus the surface gloss of this molded product was inferior and dependent on the filling time.

In addition, the surface of the molded article was observed under a microscope to find that there were many glass fibers and irregularities on the surface.

[Comparative Example 4]

A molded article was obtained in the same manner as in Example 5, except that the mold was opened to the atmosphere without connecting the gas supply device.

The surface roughness Rmax of the eased portion of the resulting molded article was 8.2 μm.

[Comparative Example 5]

A molded article was obtained in the same manner as in Example 6, except that the mold was opened to the atmosphere without connecting the gas supply device.

The surface roughness Rmax of the eased portion of the resulting molded article was 7.4 μm.

The results of Examples and Comparative Examples are shown in Tables 1 and 2 below.

According to the molding method of the present invention, the state of the mold surface can be faithfully transmitted to the molded article.

Claims (10)

A method of molding a thermoplastic resin by filling the molten thermoplastic resin into the mold, wherein the molding of the resin is carried out while reducing the solidification temperature of the surface of the resin in contact with the mold during the filling step of the resin. The method of claim 1 wherein the thermoplastic resin is an amorphous resin and the solidification temperature is a glass transition temperature. The method according to claim 1 or 2, characterized in that the thermoplastic resin is filled with a mold cavity filled with a gas having solubility in the thermoplastic resin at least twice as high as air at the solidification temperature of the resin. The method of claim 3 wherein the gas is carbon dioxide. The method according to claim 3, wherein the molding is performed by filling the molten resin into the mold cavity after the gas is present in the mold cavity under a pressure at which at least 0.1 wt% of the gas is dissolved in the resin at the solidification temperature of the resin. . 4. The method according to claim 3, wherein the molding is performed by filling a molten resin into the mold cavity after the gas is present in the mold cavity under a pressure at which 0.5% by weight or more of the gas is dissolved in the resin at the solidification temperature of the resin. . 7. A method according to any one of claims 1, 2, 4, 5 or 6, wherein the molding is injection molding. The method according to claim 4, wherein the molding is performed by filling the molten resin into the mold cavity after the gas is present in the mold cavity under a pressure at which at least 0.1 wt% of the gas is dissolved in the resin at the solidification temperature of the resin. . 5. The method according to claim 4, wherein the molding is performed by filling the molten resin into the mold cavity after the gas is present in the mold cavity at a pressure at which 0.5 wt% or more of the gas is dissolved in the resin at the solidification temperature of the resin. . 4. The method of claim 3 wherein the molding is injection molding.

Images